R1:518479a916a8

Fix broken link to PHPExcel site

phpexcel.net currently serves a 500 page, and the top Google hit for
PHPExcel (on codeplex) gives you a site warning you that it is 3 years
out of date, and to see GitHub instead.

Update the link from Maniphest's 'please install PHPExcel to enable
export' prompt.